Always go double butted, no reason to use straight gauge spokes. The WI hubs arent too popular for MTB, but they've earned an excellent reputation on the road. They're very good hubs. Only real downside is the low POE, which you may or may not care about.

If I was going to go high end, Id get a king. If I couldnt afford it, id snatch up a used king. The other hubs out there are very high quality too, but when it comes down to it its really hard to beat or come close to kings rear hubs.

Points Of Engagement. In other words, how much play the hub has before it catches. Kings have 72 clicks in a circle, WI has 24, hope has 24, low end shimanos have 16 to 18, higher end has 32 to 36. Hadley is 72 also.

I had Arch EX built with DT Swiss 240s hubs last season. The builder was Dave's Wheels and they have been great following a full season of use. Other then the pit-falls of finding the proper tire for the Arch EX rims, I'd highly recommend this setup.

POE stands for points of engagement. POE can vary greatly and it is simply a measurement of how many different freehub engagement points there are in one revolution of the wheel.
Some value POE over anything else. I'm not sure how big of a difference it makes in most situations though.
And yes, 15/14/15 would be a double butted. Double butted is simply where there is less material in the middle than on the ends. These are a bit lighter than straight gauge and a bit more elastic.